Embodiments of the present invention relate generally to characteristics display apparatuses and characteristics display methods, more particularly for indicating the characteristics of the reflection modulation of a wireless signal on an orthogonal coordinate display.
In a wireless communication system, it is in a practical use that a transmitter sends a wireless signal and a receiver of the wireless signal rectifies the received wireless signal to provide power to a semiconductor IC. Then the wireless signal from the transmitter is modulated with a signal from the semiconductor IC and reflected to communicate between the transmitter and receiver.
For example, as Japanese patent publication 10-209912 (corresponding to U.S. Pat. No. 5,784,686) discloses, in an RFID (Radio Frequency Identification) system, a reader (interrogator) transmits a message to an RF tag and then transmits a CW (Continuous Wave) carrier signal that is a non-modulated wireless signal. The RF tag conducts reflection modulation or produces a modulated reflection signal according to a response message by controlling reflection of the CW carrier signal according to the response message. The reader receives and demodulates the modulated reflection signal to obtain the response message from the RF tag. Such a system is called a backscattered RFID system.
An RF tag used in the backscattered RFID system uses ASK (Amplitude Shift Keying) as a modulation scheme to make the demodulation circuit of the RF tag simple.
FIG. 1 shows waveforms of a wireless signal RSt transmitted from a reader (interrogator) and a modulated reflection signal RSr that is a wireless signal reflected by an RF tag. To read out information of the RF tag, the reader transmits a CW carrier signal as the wireless signal RSt during time t1 through t2 as shown in FIG. 1, line A to provide necessary power to the RF tag. After that, the reader modulates a carrier signal with ASK according to commands for reading the information of the RF tag and transmits it as the wireless signal RSt. After the command transmission has finished by a time t3, the reader transmits the CW carrier signal as the wireless signal RSt to provide the power to the RF tag to be able to generate the reflection signal RSr modulated according to the response message from the RF tag.
The RF tag gets the necessary power by receiving and rectifying the wireless signal RSt transmitted from the reader. After that, the RF tag recognizes the command transmitted from the reader to generate the response message indicating requested information. The RF tag conducts reflection modulation of the wireless signal RSt according to the response message to generate the modulated reflection signal RSr as shown in FIG. 1, line B.
When evaluating the reflection modulation of the RF tag by receiving the wireless signal used in the wireless communication between the reader and the RF tag, if only amplitude dispersion of the received signal Srf that is obtained by receiving the wireless signal is relied on, there is a possibility of not obtaining the response message even when demodulating the received signal Srf.
For example, if carrier phases of the wireless signal RSt transmitted from the reader and the modulated reflection signal RSr from the RF tag are the same, an envelop waveform of the received signal Srf has a waveform of superposing an envelop waveform of the wireless signal RSt from the reader and an envelop waveform of the modulated reflection signal RSr from the RF tag as shown in FIG. 2, line A. Signal points on an IQ coordinate plane when the received signal Srf is demodulated may be located at positions P1 or P2, for example, as shown in FIG. 3A. Wherein if a modulation degree of the modulated reflection signal RSr is 100%, a distance from the origin to the position P1 indicates an amplitude level of the wireless signal RSt, and a distance from the position P1 to the position P2 indicates an amplitude level of the modulated reflection signal RSr
As described, in case of the carriers of the wireless signal RSt and modulated reflection signal RSr have the same phases, the amplitude of the envelop waveform of the received signal Srf can be an amplitude level LVa or amplitude level LVb depending on modulation status of the modulated reflection signal RSr.
On the other hand, if the phase difference between the carriers of the wireless signal RSt from the reader and the modulated reflection signal RSr from the RF tag is around π/2, the envelop waveform of the received signal Srf has no envelop waveform of the modulated reflection signal RSr as shown in FIG. 2B. Signal points in this case may be located at positions P3 or P4, for example, on the IQ coordinate plane depending on the modulation status of the modulated reflection signal RSr as shown in FIG. 3B. That is, there is an about π/2 phase difference between carriers of the wireless signal RSt and modulated reflection signal RSr so that even if the modulated reflection signal RSr is modulated with ASK, the envelop waveform of the received signal Srf does not show the modulation status of the modulated reflection signal RSr because the distance from the origin to the position P 3 and the distance from the origin to the position P4 are about the same. Therefore, if only the amplitude dispersion of the received signal Srf is used, the characteristics of the reflection modulation from the RF tag cannot be determined correctly.
The patent reference document described above completely solves a frequency null problem that occurs when a carrier frequency is swept by adding I and Q outputs. However, it cannot determine how the RF tag conducts the reflection modulation.